Important Points: Work and Energy | Science Class 9 PDF Download

Introduction

• The chapter focuses on the concepts of work, energy, and power, key elements in understanding natural phenomena.
• Energy, crucial for survival and various activities, is derived from food and other sources.

Concept of Work

• Work in science differs from its everyday usage; it requires both force and displacement.
• Scientifically, work is done when a force acts on an object and causes displacement.
• Examples: Pushing a pebble, pulling a trolley, lifting a book - all involve work as per scientific definition.
• No work is done if either force or displacement is absent.

Work Done by a Constant Force

• Work is the product of force and displacement (W = F × s).
• Work has only magnitude, not direction, and is measured in joules (J).
• Positive work: When force and displacement are in the same direction.
• Negative work: When force acts opposite to the displacement.

Energy

• Objects capable of doing work possess energy.
• Energy can be transferred between objects and transformed from one form to another.
• Types of energy: Mechanical, heat, chemical, electrical, and light energy.
• The unit of energy is joule (J), similar to work.

Kinetic Energy

• Energy due to motion is kinetic energy.
• The kinetic energy formula: where m is mass and v is velocity.
• Kinetic energy increases with speed.

Potential Energy

• Stored energy due to position or configuration is potential energy.
• Examples: Stretched rubber band, wound toy car, raised objects.
• Gravitational potential energy formula: E_p = mgh where m is mass, g is gravity, and h is height.

Law of Conservation of Energy

• Energy cannot be created or destroyed, only transformed.
• The total energy before and after transformation remains constant.
• Example: Free falling object converts potential energy to kinetic energy, but total energy remains the same.

Rate of Doing Work (Power)

• Power is the rate of doing work, measured in watts (W).
• 1 watt = 1 joule/second.
• Power depends on how fast or slow work is done.

Key Learnings

• The scientific definition of work involves force and displacement.
• Energy is the capacity to do work; it can be kinetic or potential.
• The law of conservation of energy underlines that energy transformations do not change the total energy.
• Power quantifies the rate at which work is done or energy is transformed.